Lightning Protection Design of Distributed PV System

Overview

Lightning is a phenomenon of intense discharge between thunderclouds formed by strong atmospheric convection, or between thunderclouds and the earth. Lightning is generally categorized into direct lightning and induced lightning.

Harmful Effects of Lightning

  1. Direct Lightning

Direct lightning, usually called lightning, has three major effects: thermal effect, electrical effect and mechanical effect. The lightning energy is so huge that it can instantly cause physical and electric shock damages of the struck object.

2. Induced Lightning

Induced lightning is the result of lightning strikes on surrounding area. It can cause high potential (surge voltage) and inrush (surge current) in the metal conductor in the affected area (within a radius of 2KM from the location of strikes). This is due to a wide range of electrostatic induction and electromagnetic induction phenomena caused by the accumulation of charges in the thunderclouds and the sharp decrease of charges in the thunderclouds when lightning strikes. The main damages caused by the electric potential difference are electric spark, surge current, short circuit and thus equipment damages. In particular, it does great harm to low voltage electrical systems and information systems.

The distributed photovoltaic system is mainly composed of PV array, grid connected inverter and AC distribution box.

The commonly used frame of crystalline silicon panel is aluminum alloy, and the aluminum alloy frame is connected with the metal bracket, making the PV panel vulnerable to both direct lightning strikes and induced lightning. Electrical equipments such as inverters and distribution boxes are vulnerable to induced lightning. In addition, lightning waves may intrude into buildings and endanger personal safety or damage equipment. Serious lightning attacks will cause great damage to the entire photovoltaic system.

When designing the lightning protection for a distributed photovoltaic system, the priority is to consider installing lightning rods to prevent direct lightning strikes from damaging the PV plant. At the same time, we should consider preventing induced lightning from intruding and affecting PV system as well.

Major measures for lightning protection

The distributed photovoltaic system is a three-level lightning protection structure. The lightning protection and grounding design will involve the following aspects: (Please also refer to your local standard for the installation of lightning protection systems)

 

Measure 1: Set up lightning rods to prevent low-altitude direct lightning

Measure 2: Ground PV array bracket reliably;

Measure 3: In the PV array junction box, install SPD at the input and output, and ground the chassis firmly;

Measure 4: Ground devices in equipment room firmly;

Measure 5: Add lightning isolation box at the entrance and exit of the control room, and install surge protection device to prevent induced lightning.

When the PV installation is placed on top of a building, we should take the original external lightning protection system into consideration. If the PV equipment is within its protection range, there is no need to add an external lightning protection system. Otherwise, an external lightning protection system is required. When building the lightning rod we should cover the PV equipments within its protection range and avoid shadow projection onto the PV modules at the same time.

Good grounding reduces the grounding resistance, and thus the lightning current can be easily introduced into the earth to reduce the ground potential. The grounding devices must be connected to each other through grounding bar to achieve common ground and avoid potential counteract. Independent lightning rod should be provided with a separate centralized grounding, and the grounding resistance must be less than 4Ω. Fixed metal bracket should be connected to the grounding system approximately every 10 meters.

Solar system equipment and the grounding system of the building should be connected to each other through galvanized steel with anti-corrosion and antirust treatment at the welding point. In this way the total grounding resistance can be reduced and the potential can be kept in the same level, significantly reducing the over voltage caused by lightning between different grounding points.

How to choose grounding material

Lightning rod material

For lightning rods, usually round steel with a diameter of 12-16mm is used. If a lightning protection belt is used, we should use round steel with a diameter of not less than 8mm or flat steel with a thickness of not less than 4mm. The height of the lightning rod above the protected object should be greater than or equal to the horizontal distance from the lightning rod to the protected object. The higher the lightning rod, the larger its protection range.

Grounding material

Grounding body should be hot-dip galvanized steel. Its specifications are generally: steel pipe with a diameter of 50mm, thickness not less than 3.5mm; 50mm*50mm*5mm angle steel, length not less than 2.5m; or flat steel of 40mm*4mm, length is generally 2.5-4m. The buried depth of the horizontal flat steel grounding body should be no less than 0.5m and the buried depth of the vertical angle steel no less than 2.5m, and the welded joints should be treated again with anti-rust and anti-corrosion coating.

Wire conductor material

It is recommended to use hot-dip galvanized round steel or flat steel for the wire conductor. Round steel should be preferred, and its diameter should be no less than 8mm. If flat steel is used, the thickness should be no less than 4mm. A double-layer insulated multi-strand copper wire with a cross-sectional area of 35 sqmm should be used when there is higher requirement.

Equipotential connection

The aluminum alloy or galvanized frame of PV module is coated and clamping can not meet the grounding requirements. Only when the grounding hole of PV module is connected to the bracket, will the PV module be effectively grounded. Therefore, a direct equipotential bonding between the external lightning protection system and the PV module must be established at these locations.

Surge protection

To reduce the damage caused by surge and lightning overvoltage, a surge protector should be installed on the live cable and the following protective measures needs to be taken for the PV system:

1. Add surge protection device to the DC input and AC output of the inverter.

2. Install a surge protector in the grid power distribution box (distribution cabinet) to protect lightning waves from entering the connecting cable. In order to prevent short-circuiting when the surge protector fails, a circuit breaker or fuse must be connected in series with the surge protector. The rated current of the circuit breaker (fuse) cannot be greater than the rated value of over current protection recommended by the surge protector product specification.

Summary

In large-scale ground solar plant, it is necessary to build a complete lightning protection system, including lightning rods, lightning protection belts, grounding devices, grounding cables, installing SPD on equipment. For industrial and commercial projects, original lightning protection devices of the building can be used to achieve lightning protection and grounding of the equipment. While for residential projects, if the house is not that high, and there are higher buildings in surrounding area, then the requirements for lightning protection are relatively low.

 

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